1. Field of the Invention
The present invention generally relates to an image forming apparatus and, more particularly, to an image forming apparatus having a plurality of developing units selectively operated to develop, for example, different color component images.
2. Description of the Related Art
Japanese Laid-Open Patent Application No. 64-971 discloses a technique for selectively driving a plurality of developing units by using a single drive power source. A power generated by the drive power source is transmitted to each of the developing units via a train of gears and a clutch provided to each of the developing units. Accordingly, one of the developing units can be selectively operated by operating the clutch.
According to this technique, since a power of the drive power source is transmitted to each clutch, a complex train of gears is needed and a space for accommodating the gears is needed. Accordingly, this technique is not appropriate for reducing size of an image forming apparatus using this technique.
Japanese Laid-Open Patent Application No. 4-280273 discloses a mechanism for selectively operating one of a plurality of developing units. This mechanism includes a drive gear which is driven by a single drive power source and four shift gears connected to respective developing units. Each of the shift gears is disengageably engaged with the drive gear by means of a pressing cam so that the shift gears are selectively engaged with the drive gear.
According to the above-mentioned mechanism, since the four shift gears are selectively engaged with the drive gear, a complex mechanism is needed for operating the pressing cam.
Additionally, Japanese Laid-Open Patent Applications No. 5-249833, No. 5-216337. No. 8-179621 and No. 5-333701 disclose developing apparatuses having a plurality of developing rollers such as a first developing roller for developing a latent image on a photosensitive drum by a first component color and a second developing roller for developing a latent image by a second component color. In these apparatuses, when a developing operation is switched from one of the developing rollers to another one of the developing rollers, a bristle cutting operation is performed by reversing the developing roller after the developing operation by the one of the developing units is completed.
However, the above-mentioned patent documents do not refer to a mechanism for driving each of the developing rollers. If each of the developing rollers has an individual drive power source, the size of the developing apparatuses is increased which results in an increase in the size of an image forming apparatus using such a developing apparatus. Accordingly, it has been suggested to use a common drive power source for the developing rollers.
When such a mechanism in which a power is selectively transmitted from a single common drive power source to a plurality of developing rollers is used, an engagement of a power transmitting gear is switched from one of the developing rollers to another one of the developing rollers. When a gear is engaged with another gear, the engagement can be smoothly performed when a top of a tooth of one of the gears faces a bottom of a tooth of the other one of the gears. On the other hand, if a top of a tooth of one of the gears faces a top of a tooth of the other one of the gears, it may be difficult to make a smooth engagement since the tops of the teeth run against each other. Accordingly, if such a condition occurs in the above-mentioned developing apparatus, a smooth switching operation of the driving mechanism of the developing rollers cannot be performed.